How to bake high to low poly normals for multiple objects on one normal map?
It's actually very simple. no need for weird ways.
Simply uncheck "clear" (in 2.7) or "clear image" (in2.8) in your bake settings, then bake each object at a time, assuming they don't share UV space.




/!\ watch out for the margin, make sure to set it low enough it doesn't overlap with an other UV island